There is known a projection-type image display apparatus wherein incoherent light emitted from a light source such as a halogen lamp or a high-pressure mercury lamp is projected onto a planar image display device such a liquid crystal light bulb, and light emitted from the image display device is enlarged and projected onto a screen by a projection lens for thereby displaying an image.
Since the projection-type image display apparatus uses incoherent light, it is problematic in that it consumes much electric power and the displayed image has a low luminance level. Furthermore, since the wavelength range of the incoherent light emitted from the light source is wide, it has been difficult to increase a chromaticity range. The projection-type image display apparatus cannot easily be reduced in size because the planar image display device is used as an image display device. In addition, inasmuch as the displayed image is out of focus unless the displayed image is projected within the depth of focus of the projection lens, the user is required to adjust the focus of the displayed image depending on the position of the screen, and hence the projection-type image display apparatus is not user-friendly.
A projection-type image display apparatus which employs a laser beam source for emitting a laser beam have been proposed or developed as a technology for solving the above problems. Such a projection-type image display apparatus includes a scanning-type image display apparatus for displaying an image by projecting a laser beam emitted from a light source onto a screen while the screen is being scanned by the laser beam.
The projection-type image display apparatus with the laser beam source is disadvantageous in that the quality of displayed images is low because of visually unpleasant noise called speckles on the screen. Speckles refer to spot-like noise produced by the interference of light scattered from spots on the screen when a coherent beam such as a laser beam is projected onto the screen.
Technologies which are capable of reducing such speckles include a two-dimensional image display apparatus disclosed in Patent document 1 and a light emission apparatus disclosed in Patent document 2.
According to the above technologies, the polarized state (the direction of polarization and the type of polarization) of a laser beam emitted from a light source is randomly changed, and the laser beam in the changed polarized state is projected onto a screen. Since the pattern of speckles varies with time, it is possible to average the speckles over time, resulting in reduced speckles.